1. Field of the Invention
The present invention relates to a process for the preparation of polymers of .alpha.-olefins, especially homopolymers of ethylene and copolymers of ethylene and higher .alpha.-olefins. In particular the invention relates to a solution process for the preparation of polymers of .alpha.-olefins in which .alpha.-olefin monomer is polymerized in the presence of a coordination catalyst capable of being used at relatively high polymerization temperatures, especially temperatures above 150.degree. C.
2. Description of the Prior Art
Polymers of ethylene, for example, homopolymers of ethylene and copolymers of ethylene and higher .alpha.-olefins, are used in large volumes for a wide variety of end-uses, for example, in the form of film, fibres, moulded or thermoformed articles, pipe, coatings and the like.
Processes for the preparation of homopolymers of ethylene and copolymers of ethylene and higher .alpha.-olefins are known. Such processes include processes in which the monomers are polymerized in the presence of a coordination catalyst, for example, a catalyst comprising a compound of a transition metal belonging to Groups IVB-VIB of the Periodic Table and an organometallic compound of a metal belonging to Groups I-IIIA of the Periodic Table.
There are two types of processes for the manufacture of polyethylene that involve the polymerization of monomers in the presence of a coordination catalyst viz. those which operate at temperatures below the melting or solubilization temperature of the polymer and those which operate at temperatures above the melting or solubilization temperature of the polymer. The latter are referred to as "solution" processes.
In processes operated below the melting or solubilization temperature of the polymer i.e. low temperature processes, ethylene is converted to solid polymer which remains suspended in an organic solvent as a "slurry" or is fluidized in a gaseous monomer stream. In general the molecular weight of the polymer is controlled by the use of hydrogen. Substantial amounts of hydrogen may be required. Preferred catalysts for the low temperature polymerization processes tend to exhibit high catalytic activity, not to adhere to the surface of the polymerization reactor and to yield polymer of high bulk density and of relatively low molecular weight, thereby lowering the need for hydrogen. In addition the ability of the catalyst to produce polymer in the form of regular and uniform particles may be an advantage.
A preferred process for the polymerization of .alpha.-olefins is the high temperature or "solution" polymerization process, an example of which is described in Canadian Patent No. 660,869 of A. W. Anderson, E. L. Fallwell and J. M. Bruce, which issued Apr. 9, 1963. In a solution process the process is operated so that both the monomer and polymer are soluble in the reaction medium. Under such conditions accurate control over the degree of polymerization, and hence the molecular weight of the polymer obtained, is achieved by control of the reaction temperature. It is believed that the termination reaction controlling the molecular weight of the polymer is highly dependent on temperature. In an embodiment of a solution process, the molecular weight of the polymer may be further controlled through the use of relatively small amounts of hydrogen, as is described in Canadian Pat. No. 703,704 of C. T. Elston, which issued Feb. 9, 1965.
There are a number of advantages to a solution polymerization process, for example, the ability to control the molecular weight of the polymer obtained, the ability to operate the process as a continuous process and to recover the polymer by precipitation without the need for washing, the efficient use of catalyst, the properties of the polymer obtained and the possibility of efficient use of energy.
A disadvantage of a solution process is that part of the catalyst remains in the polymer of ethylene. Such catalyst, which may be referred to herein as "catalyst residue", may contribute to degradation of the polymer during subsequent processing of the polymer e.g., in extrusion, injection moulding and the like, and/or on exposure of fabricated articles to ultra violet light. The amount of catalyst residue is related, at least in part, to the overall activity of the catalyst employed in the polymerization step of the process as the higher the overall activity of the catalyst the less catalyst that is, in general, required to effect polymerization at an acceptable rate. Catalysts of relatively high overall activity are therefore preferred in solution polymerization processes.
Two important factors in determining the overall activity of a catalyst are the instantaneous activity of the catalyst and the stability of the catalyst under the operating conditions, especially at the operating temperature. Many catalysts that are stated to be very active in low temperature polymerization processes also exhibit high instantaneous activity at the higher temperatures used in solution processes. However such catalysts tend to decompose within a very short time in a solution process and thus the overall activity is disappointingly low. Such catalysts are of no commercial interest for solution processes. Other catalysts may exhibit acceptable overall activity at the higher temperatures of a solution process but show tendencies to yield polymers of broad molecular weight distribution or of too low a molecular weight to be commercially useful for the manufacture of a wide range of useful products. The requirements for and the performance of a catalyst in a solution polymerization process are quite different from those of a catalyst in a low temperature polymerization process, as will be understood by those skilled in the art.
A process for the preparation of polymers of ethylene at high temperatures in which the catalyst is a coordination catalyst consisting of titanium tetrachloride, a vanadium compound and aluminum trialkyl and capable of being used in a solution process is described in Canadian Pat. No. 635,823 of D. B. Ludlum, N. G. Merckling and L. H. Rombach, which issued Feb. 6, 1962.
The preparation of a heat-treated polycatalyst particularly adapted for the low temperature polymerization of propylene is described in Canadian Pat. 787,408 of R. H. Jones, which issued June 11, 1968. The catalyst is prepared by adding to a halide of a transition metal in its highest valence state the amount of an organometallic compound required to reduce the valence state of the transition metal by one, heat treating the admixture so obtained to form the violet form of titanium trichloride and then activating the resultant product with additional organometallic compound.
It is believed that the active species of a titanium-containing coordination catalyst is a reduced form of titanium, especially in the form of titanium trichloride. G. Natta et al discuss the crystalline modifications of titanium trichloride in J. Poly. Sci 51 399-410(1961). The performance of titanium trichloride as a catalyst species appears to relate to its crystal form, crystal size and size distribution, adsorbed species and the like which in turn depend on the method used in the preparation of the catalyst. A commercially available form of a titanium trichloride polymerization catalyst is TiCl.sub.3.1/3 AlCl.sub.3 which is available from Stauffer Chemical Co. of Westport Conn., U.S.A., under the trade designation "TiCl.sub.3 AA". The violet form of titanium trichloride may be formed in the reactor of a solution polymerization process, especially when the temperature in the reactor is above 150.degree. C.